With the application 31P-magnetic resonance spectroscopy to living tissue, it has been established that the cellular pH of tumors and of normal tissue is similar, whereas the extracellular pH differs between these tissues. The overall objective of this research is to exploit the resulting difference in the cellular pH gradient between tumor and normal tissue, for the treatment of cancer. Theory predicts, and in-vitro studies confirm, that the cell pH gradient markedly influences the intra-extracellular distribution of drugs exhibiting weak acidic or basic properties. The clinical relevance of this finding is profound. Analysis of published data reveals that the cell pH gradient substantially differs between tumor and normal tissue in humans, and additionally, the magnitude of this difference increases with increasing radial distance from supplying vessels. This study determines if the pH gradient governs the cytotoxicity of weak electrolytes in the complex and dynamic tumor microenvironment, and therefore whether the pH gradient difference between tumor and normal tissue can be exploited for the treatment of cancer. To evaluate this hypothesis, tumor growth delay induced by a weak acid chemotherapeutic will be compared to that induced by a weak base, under normal and pH gradient modified conditions. If the pH gradient dependent hypothesis is operative in-vivo, a pH gradient change will have opposite effects on the cytotoxicity of these two weak electrolytes. Second, the magnitude of tumor growth delay following pH gradient modification will be evaluated for electrolytes bearing two vs one ionizable group. If operative, pH gradient dependent drug uptake will lead to substantially different tumor growth delay changes for the two electrolytes. A third aim determines if pH gradient modification influences drug cytotoxicity in tumor cells most distal from functional vasculature, and if the magnitude of this effect is greater or less than pertains to cells more proximal to functional vasculature. Importantly, comparing the effects of a weak acid vs base, will preclude the masking or mimicking of any pH gradient effect, secondary to induced changes in tumor blood flow or resulting consequential effects on the tumor microenvironment. Confirmation of the hypothesis proposed in this application will provide a basis for the design and utilization of drugs which preferentially accumulate in the intracellular compartment of tumor tissue, including those tumor regions distal from supplying vessels, which are exposed to the lowest concentration of systemically administered drugs, and which are likely to be increasingly radioresistant.